Field of the Invention
The present invention relates to an organic light emitting display device, and more particularly, to an organic light emitting display device with low reflection.
Discussion of the Related Art
Recently, as the importance for information displays has been on the rise, and the demand for using portable information mediums has increased, research into and commercialization of lighter, thinner flat panel displays (FPDs) to replace cathode ray tube (CRT) displays and other existing display technologies have been actively conducted. In the flat panel display field, liquid crystal displays (LCDs) have come to prominence. However, because LCDs are light receiving devices rather than light emitting devices and have shortcomings in terms of brightness, contrast ratio, viewing angle, and the like, novel display technologies to overcome such shortcomings have been actively developed.
An organic light emitting display device, one of these display device technologies, is self-luminous and, thus, is excellent in terms of a viewing angle and a contrast ratio. Also, because an organic light emitting display device does not require a backlight, the organic light emitting display device may be lighter and thinner and is advantageous in terms of power consumption. In addition, the organic light emitting display device may be driven by a low DC voltage with a fast response speed.
Hereinafter, a basic structure and operational characteristics of an organic light emitting display device will be described in detail with reference to the accompanying drawings. FIG. 1 is a diagram illustrating a light emitting principle of a general organic light emitting diode.
In general, an organic light emitting display device includes an organic light emitting diode (OLED), as illustrated in FIG. 1. The OLED includes an anode 18 as a pixel electrode, a cathode 28 as a common electrode 28, and organic layers 30a, 30b, 30c, 30d, and 30e formed between the anode 18 and the cathode 28.
The organic layers 30a, 30b, 30c, 30d, and 30e include a hole transport layer (HTL) 30b, an electron transport layer (ETL) 30d, and an emission layer (EML) 30c interposed between the hole transport layer 30b and the electron transport layer 30d. Here, to enhance luminous efficiency, a hole injection layer (HIL) 30a is interposed between the anode 18 and the hole transport layer 30b, and an electron injection layer (EIL) 30e is interposed between the cathode 28 and the electron transport layer 30d. 
In the OLED configured in such a manner, when a positive (+) voltage and a negative (−) voltage are applied to the anode 18 and the cathode 28, respectively, holes passing through the hole transport layer 30b and electrons passing through the electron transport layer 30d are transferred to the emission layer 30c to form excitons. When the excitons transit from an excited state to a ground state, namely, a stable state, light is generated.
In the organic light emitting display device, sub-pixels each including the OLED having the foregoing structure are arranged in a matrix form and selectively controlled with a data voltage and a scan voltage to display an image. Here, the organic light emitting display device is classified into a passive matrix type organic light emitting display device or an active matrix type organic light emitting display device using a thin film transistor (TFT) as a switching element. Here, in the active matrix type organic light emitting display device, a TFT, an active element, is selectively turned on to select a sub-pixel and light emission of the sub-pixel is maintained with a voltage maintained in a storage capacitor. In such a general organic light emitting display device, a circular polarizer is applied to an upper surface of a panel assembly to reduce reflection due to various types of wires or electrodes formed of a metallic material.
FIG. 2 is a view exemplarily illustrating a structure of a general organic light emitting display device, and FIG. 3 is a view exemplarily illustrating another structure of a general organic light emitting display device.
As shown in FIGS. 2 and 3, a circular polarizer including a quarter wave plate (QWP) 62 and a linear polarizer 63 is applied to an upper surface of a panel assembly 2 to reduce reflection. A protection layer 66 may also be provided on a top surface.
In the related art organic light emitting display device, visibility is reduced outdoors (e.g., under sunlight conditions), and reflectance is increased due to the organic light emitting diodes, as well as various types of wires or electrodes. Further, wires, electrode patterns, or the like may be seen. To solve such a problem, a circular polarizer is applied.
That is, if the quarter wave plate 62 is arranged on the panel assembly 2 such that its optical axis forms an angle of 45° with a transmission axis of the linear polarizer 63, external light will be reflected from the inside of the panel assembly 2. The reflected light is discharged to the outside in a direction perpendicular to the transmission axis of the linear polarizer 63. This may lead to lower reflectance. For reference, reflectance is a function of refractivity and is increased as a refractivity ratio is increased. Air has refractivity of 1, and glass has refractivity of 1.5. Thus, when light is incident onto a front surface of the glass from air, about 4% of the incident light is reflected.
However, in the structure of FIG. 2, brightness in the organic light emitting display device of the related art is reduced by at least 50%. That is, transmittance (transmissivity, transmission factor) of the linear polarizer 63 is about 40%˜50%, and light generated from the organic light emitting diode has its brightness reduced to about 40%˜50% after passing through the circular polarizer.
As shown in FIG. 3, in a case where a dual brightness enhancement film (DBEF) 67 is applied between the quarter wave plate 62 and the linear polarizer 63, brightness may be enhanced but reflectance may be also increased as a trade-off.
Further, a sunglasses effect is not considered in the related art. That is, in a case in which a transmission axis of polarized sunglasses worn by the user is perpendicular to the transmission axis of the linear polarizer 63 of the organic light emitting display device, a user may not be able to view the display being driven because the screen appears to be black.